History of Puberty: Normal and Precocious

The youngest documented case of central precocious puberty (CPP) was that of a Peruvian girl named Lina Marcela Medina de Jurado, who gave birth to a healthy baby boy in 1939 when she was 5 ½ years old [1]. Lina’s parents brought her to the hospital because of an increasing abdominal mass that was assumed to be a tumor. There, she was discovered to be more than 7 months pregnant and underwent a cesarean section 6 weeks later. She had reportedly reached menarche at 8 months of age and during her cesarean was noted to have fully mature reproductive organs. Her case was presented at the annual meeting of Academia Nacional de Medicina del Peru to great interest and bewilderment. At the time, there was only the most nascent understanding of normal reproductive physiology, without awareness of potential perturbations in the timing of hypothalamic-pituitary-gonadal (H-P-G) axis activation such as occurs in CPP. In this review, we aim to highlight examples of key historical milestones elucidating fundamental aspects of both normal and precocious pubertal development. We will review the evolution and expansion of therapeutic strategies for the treatment of children with CPP and provide a brief description of currently available pharmacologic agents.

Testicular function has been recognized for centuries as being required for male pubertal maturation and maintenance of secondary sex characteristics, as illustrated by castration of male animals, male eunuchs, and choir boy castrati in medieval and renaissance times. (This practice was banned by Pope Sixtus V in 1587 but continued illicitly for centuries [2].) The endocrine nature of testicular function began to be understood in the 19th century through scientific experimentation by Arnold Berthold [3], who transplanted testes from roosters into capons (castrated male chickens) and noted increases in male-typical characteristics. The scientific approach devolved over subsequent decades into “organotherapy,” most notoriously illustrated by efforts of Charles-Edouard Brown-Sequard to popularize testicular extracts as a rejuvenation treatment for men [4] and by promotion of xenotransplantation of animal testes into humans [2]. In the early part of the 20th century, this approach gave the budding field of endocrinology a reputation as the domain of charlatans, and it was even called “endocriminology [5].”

The 1920s and 1930s were an era of rapid scientific progress, ushering in the discovery and synthesis of numerous gonadal steroid hormones. A bioassay using the change in surface area of a capon’s comb allowed quantification of androgen activity by 1929 [6]. Soon afterward, the isolation of androsterone by Adolf Butenandt in 1931 paved the way for discovery of more potent androgens. Recognizing the commercial potential of a purified androgen, the pharmaceutical companies Ciba and Schering-Kahlbaum cooperatively shared their scientists’ respective discoveries to accelerate bringing a product to market. One of these scientists was Leopold Ruzicka from the Swiss Federal Institute of Technology, most of whose early career was devoted to perfume research. Independently, Ernst Laquer at the University of Amsterdam and Organon Pharmaceuticals used 100 kg of bull testes to isolate 10 mg of a steroid they named “testosterone,” which was shown to have more androgenic activity than androsterone. This group was the first to report the structure of testosterone [7], but the Ciba/Schering-Kahlbaum consortium rapidly followed with their own reports of the same compound and approaches to its synthesis [8, 9].

In the early years of the 20th century, it was known that the ovary was critical for female reproductive development and fertility, and endocrine factors were understood to mediate ovarian function. In 1923–1924, Edgar Allen and Edward Doisy at Washington University in St. Louis (who were neighbors and drove to work together) [10] showed that follicular fluid obtained from sow ovaries was able to induce cornification of the vaginal epithelium in ovariectomized mice as well as precocious sexual maturation in young mouse pups [11]. The active substance in the follicular fluid was not known, but numerous names came into use in the next few years, including folliculin, progynon, equlin, and theelin. The term “estrogen” had been coined previously as an overarching term because the substances induced estrus-like changes in female mammals. In 1929, Allen and Doisy [12] were able to isolate the hormone later known as estrone from sow follicular fluid, human placenta, and the urine of pregnant women. Separately, Adolf Butenandt [13] (of androsterone fame) and colleagues obtained an oil extract of estrone from sow ovaries. The same year, working in London, Guy Marrian extracted a steroid later called estriol from human pregnancy urine [14]. Estrone and estriol were found to be chemically interconvertible in 1931 by Butenandt, demonstrating that both compounds were structurally similar variations of the steroid backbone [15]. Estradiol, the most potent of the ovarian hormones, was first produced by chemical reduction of the keto group of estrone, but it was not identified in nature until 1935, when MacCorquodale, Thayer, and Doisy [16] isolated it from sow ovaries. These researchers processed 8,000 pounds of sow ovaries to obtain 11 mg of the substance. The nomenclature issue was resolved in 1932, when the League of Nations sponsored the International Standardization of Sex Hormones. The variable nosology of female sex hormones was then standardized to the currently used system of estrone and estriol, with the addition of estradiol to the roster after its discovery.

For his work on sex hormones, Adolf Butenandt shared the 1939 Nobel Prize in Chemistry with Leopold Ruzicka, for methods to mass produce testosterone. Edward Doisy was awarded the Nobel Prize in Physiology or Medicine in 1943, but this was for his later discovery of vitamin K (Fig. 1).

The role of the hypothalamus and pituitary gland in the control of the ovary was not fully realized until the 1920s [17]. In 1926, Philip Smith [18] published his studies of the effects of pituitary ablation in the rat, showing that this led to ovarian atrophy. When ablation was followed by transplantation of fresh anterior pituitary tissue, the ovaries and uterus grew. If anterior pituitary tissue from an adult rat was transplanted into a juvenile rat, precocious sexual maturation resulted. The nature of the pituitary secretions required for this phenomenon was worked out soon afterward when Bernhard Zondek [19] in Berlin identified two activities contained in blood and urine from post-menopausal women, which he named prolan A (follicle stimulating) and prolan B (luteinizing). Interestingly, Bernhard Zondek’s brother, Hermann Zondek, was an endocrinologist who conducted pioneering studies of thyroid function and was also the personal physician for two German chancellors before the rise of Nazi Germany. The separation of the two ovarian stimulating activities followed soon after Zondek’s report, when Frederick Hisaw at the University of Wisconsin, chemist H.L. Fevold, and Samuel Leonard, then a graduate student, isolated extracts containing luteinizing hormone and follicle stimulating hormone from the anterior pituitary glands of hogs and showed their distinct actions on the ovary when administered to rats [20]. It was not for another 10 years, however, that LH and FSH were purified [17].

Although the endocrine activities influencing follicle development and luteinization were known in the early 1930s, the regulation of these functions was unclear. A role for the hypothalamus was suspected as it was already known that hypothalamic nerve fibers passed down the pituitary stalk to connect with the posterior pituitary. Because no neural connection between the hypothalamus and the anterior pituitary had been found, some researchers postulated that nerve impulses traveled from the hypothalamus to the superior cervical ganglion and then on to the anterior pituitary. Some also considered that higher brain centers controlled anterior pituitary function directly. Working in Cambridge, England, Geoffrey Harris investigated the role of the hypothalamus in ovarian function by electrically stimulating the hypothalamus in rabbits. This stimulation led to ovarian follicle maturation and ovulation within 40 h. Because of technical challenges, he could not perform this procedure on animals following pituitary stalk transection and thus could not show the role of the stalk in transmission of hypothalamic stimuli [21]. His conclusion at the time was that hypothalamic neural impulses likely traveled down the pituitary stalk to stimulate the posterior pituitary, which then transmitted input to the anterior pituitary, either through connecting nerve fibers or possibly hormonal effects. However, later studies revealed that direct electrical stimulation of the pituitary had no effect.

Knowing that the brain regulates the pituitary-ovarian axis, for the next 15 years it remained uncertain how this connection actually happened. During this interval, Geoffrey Harris [22] continued his electrical stimulation studies, but also became interested in the hypothalamic-pituitary portal system as a potential route of signal transfer, confirming in living animals that the direction of blood flow was toward the pituitary. Harris, working with Swedish investigator Dora Jacobsohn [23], then conducted a series of transplantation studies in rats. In these studies, they placed fresh anterior pituitary tissue into hypophysectomized female recipients, locating the tissue either inferior to the hypothalamus or in the subarachnoid space beneath the temporal lobe. In both groups, the transplanted tissue revascularized. However, only those animals whose sub-hypothalamic transplants were revascularized by the portal system had normal ovarian function, including normal estrus, mating behavior, and litter production. Male recipients had similar successful reproductive outcomes [23]. These animals also had normal adrenal and thyroid development, unlike the recipients of tissue placed under the temporal lobes, which had ovarian, adrenal, and thyroid gland atrophy. These studies confirmed that the hypothalamus regulates the pituitary through humoral releasing factors, including control of the gonadal, adrenal, and thyroid axes. Interestingly, female recipient rats produced milk but were unable to nurse the pups unless they received injections of oxytocin. This showed that oxytocin is not under humoral regulation, consistent with the known hypothalamic innervation of the posterior pituitary. Because some of the donor pituitary tissue came from 2- to 10-day old rats, yet provided adult levels of ovarian stimulation, Harris reasoned that the onset of puberty was not simply a matter of pituitary or ovarian maturation. In 1955, he proposed that the timing of puberty in mammals depends not on pituitary maturation but on hypothalamic maturation [24]. The nature of the postulated hypothalamic releasing factors remained unknown and was the focus of intense research efforts by several groups over the next 20 years. In fact, the concept that the brain – the center of cognition – also secreted hormones was incredible to some. In the latter half of his career, Harris’s work included attempts at isolating the hypothalamic factors that stimulated ovarian function, bringing him into competition with Roger Guillemin and Andrew Schally [25].

Roger Guillemin was born in Dijon, France, in 1924, and Andrew Schally in Wilno, Poland, in 1926. Both men endured hardships in post-WW II Europe before emigrating to Montreal, Canada, for scientific training. Although Guillemin had a medical degree, the focus of both their careers was the physiology of the hypothalamic-pituitary interaction. Although they were colleagues for a time at Baylor University in Houston and published papers together, their careers diverged geographically, with Guillemin settling at the Salk Research Institute in La Jolla, CA (Fig. 2) and Schally at Tulane University in New Orleans. An intense competition developed between the two, with each building a large, well-funded multidisciplinary research team dedicated to identifying the hypothalamic releasing factors postulated by Geoffrey Harris. The interactions between the groups at scientific meetings were often acrimonious, with refusal to share materials and data. Both teams identified the structure of thyrotropin releasing hormone virtually simultaneously in 1969 before turning their attention to GnRH, which was then known as the luteinizing hormone releasing factor. Schally’s group was the first to publish the amino acid sequence of GnRH in 1971 [26] following isolation of the substance (800 µg from 160,000 sheep hypothalami), identification of its amino acid sequence, synthesis of the decapeptide, and physiologic demonstration of its effect on LH secretion using the relatively new technique of radioimmunoassay. Their publication was followed 2 months later by the confirmatory report from Guillemin’s group [27]. In recognition of the accomplishments of both investigators, they received the Nobel Prize in Physiology or Medicine in 1977 “for their discoveries concerning the peptide hormone production of the brain.” Interestingly, they shared the prize with Rosalyn Yallow, who received the award for her work on radioimmunoassays for peptide hormones. Schally and Guillemin’s discoveries set the stage for the development of GnRH analogs.

Previous studies of gonadotropin levels over the course of the day in ovariectomized laboratory animals and humans had shown apparently random fluctuations. Investigating this phenomenon, Dierschke et al. [28] in 1970 sampled peripheral LH concentrations every 10–30 min in ovariectomized monkeys and identified striking hourly rhythmic fluctuations. Because this pattern was unlikely to be caused by rapid changes in metabolic clearance and the elimination half-life was identical to injected LH, they correctly attributed this phenomenon to pulsatile LH secretion. Although pulsatile GnRH secretion was later identified in 1979 [29], the linkage of GnRH pulses to LH pulses was not conclusively shown until 1982. In that year, Clarke and Cummins [30], working in Melbourne, Australia, implanted needles near the pituitary stalk in sheep. They used a stylet passed though one needle to lance the portal vessels and used the other needle to withdraw the resulting blood for GnRH assay, sampling every 5–15 min with simultaneous measurement of systemic LH from an indwelling jugular venous catheter. There was a consistent and tight temporal relationship between GnRH and LH pulses [30]. Although the secretory relationship was now known, the origin of the pulsatility would remain a mystery for decades.

With the discovery of GnRH came interest in using this knowledge for diagnostic and therapeutic applications. There was a rapid proliferation of analogs that not only helped explore structure-function relationships but also manipulated the agonist activity and metabolic half-life. Much of this work was done by Wylie Vale at the Salk Institute in conjunction with Roger Guillemin [31] as well as David Coy [32] working with Andrew Schally in New Orleans. It was found that substitution of L-glycine at position 6 of the decapeptide with a D-isomer amino acid and deletion of the terminal glycine-10 plus the addition of an N-ethylamide group to proline-9 both increased secretion of LH in cell culture and intact rats [31]. The investigators postulated that this phenomenon was due at least in part from interference with known hydrolysis sites between positions 6–7 and 9–10, thus prolonging the half-life (Fig. 4). Other substitutions created antagonists. Importantly, these animal studies were conducted over relatively short time frames of <24 h duration. Excitement grew that the use of GnRH agonists would be a treatment for infertility and that GnRH antagonists could act as fertility-control agents [33].

This concept of how GnRH analogs could be applied to clinical medicine was altered by a seminal discovery in 1978. Belchetz et al. [34], in the laboratory of Ernst Knobil, studied monkeys with hypothalamic lesions that eliminated endogenous GnRH secretion. The investigators found that, contrary to expectations, continuous infusion of GnRH did not cause resumption of pituitary gonadotropin release. However, when GnRH was infused in a pulsatile pattern, with doses given hourly, LH and FSH secretion was restored. Further, when the investigators superimposed continuous infusion on top of pulsatile administration, gonadotropin secretion was downregulated, resulting in suppression of the H-P-G axis [34]. This work laid the foundation for the development of gonadotropin releasing hormone analogs (GnRHas) as a therapeutic class that would find wide applicability in many areas of medicine.

Kisspeptin is the product of the KISS1 gene. It was first discovered by Lee et al. [35] in 1996 when its cDNA was produced from a melanoma cell line and identified as an anti-metastatic gene that suppressed metastasis by 95%. The gene name came from a combination of the laboratory group’s location in Hershey, PA (home of The Hershey Company), and the laboratory’s interim nomenclature for Suppressor Sequences, in acknowledgment of the iconic chocolate candy. Five years later, in a separate search for the natural ligand of the orphan G-protein receptor GPR54, Kotani et al. [36] detected an unknown binding activity in a human placental extract. Using sequential HPLC fractionations, they purified a series of peptides from the extract that bound to GPR54 and that they identified as products of the KISS1 gene by tandem mass spectrometry. GPR54 expression was localized in the human brain, pituitary, and spinal cord. In cell models expressing GPR54, kisspeptin exposure initiated multiple intracellular signaling cascades, and during in vivo experiments it was shown to stimulate oxytocin release in mature female rats. However, the link between the kisspeptin/GPR54 system and reproductive neuroendocrinology was not established until 2003, when two reports were published 2 months apart showing GPR54 mutations in families with hypogonadotropic hypogonadism [37, 38]. This relationship was reinforced by the finding that intravenous infusions of kisspeptin in healthy male humans led to increases in circulating LH, FSH, and testosterone concentrations [39]. Over the ensuing years, work by multiple investigators localized kisspeptin-producing neurons in the arcuate nucleus and the anteroventral periventricular nucleus of the hypothalamus in rodents and the arcuate nucleus and preoptic area in primates. Central administration of kisspeptin stimulated GnRH neurons, causing GnRH release, and kisspeptin neurons were found to have close physical connections with GnRH neurons. In 2007, Goodman et al. [40] discovered that kisspeptin neurons in the arcuate nucleus co-expressed the stimulatory neurotransmitter neurokinin B and the inhibitory neurotransmitter dynorphin A and termed them KNDy neurons.

Despite the known pulsatile secretion of GnRH, the existence and location of the putative pulse generator was unknown until recently. It was initially postulated that an interconnected cluster of GnRH neurons could produce its own regular pulses [41]. Cell culture studies, however, failed to show spontaneous synchronization between neurons. Further, optogenetic studies, in which a light-sensitive protein was expressed in GnRH neurons in vivo and the cells stimulated by exposure to blue light, did not show LH secretion when GnRH neuron bursts were experimentally synchronized [42]. Mounting evidence also showed that GnRH pulsatility arises from locations extrinsic to GnRH neurons. Careful fine mapping of hypothalamic electrical activity in monkeys revealed a close correlation between LH secretion and electrical activity in the arcuate nucleus [43]. Co-expression of stimulatory and inhibitory neurotransmitters in the arcuate KNDy neurons suggested the possibility of autocrine or paracrine cyclicity and made them clear candidates for the pulse generator. Using sophisticated techniques in vivo to study individual cells within the arcuate nucleus of the mouse, Clarkson et al. in 2017 [44] showed that intracellular KNDy neuron calcium fluctuations, which are associated with cellular electrical activity, correlated nearly perfectly with LH pulsatility. In vivo optogenetic techniques allowed the same investigators to specifically stimulate KNDy neurons, which led to pulse-like increases in LH. Similar approaches permitted an active suppression of KNDy neurons that caused decreased LH secretion. Further studies [45] by other investigators in large mammals such as sheep and goats support the importance of arcuate KNDy neurons in pulse generation. Taken together, these studies show that the arcuate nucleus KNDy neurons are the hypothalamic GnRH pulse generator, critical for regulation of gonadotropin and gonadal steroid secretion [46], finally settling a question that had been unanswered since 1970.

Thus, decades of basic research into reproductive physiology, starting with an early understanding of gonadal hormones and proceeding to pituitary regulation and then hypothalamic control, proved critical to understanding how puberty unfolds. Furthermore, these discoveries were essential for clinical advances and led directly to major therapeutic interventions.

The delineation of the components and hormonal products of the H-P-G axis afforded tremendous insight into the normal physiology of puberty. However, there was no standardized approach to assess clinical pubertal status and progression until the introduction of the Tanner stages by Dr. James Tanner [47] in the 1960s. James M. Tanner was a British pediatric endocrinologist and Professor Emeritus at the Institute of Child Health at the University of London. From 1948 to ∼1968, he oversaw a study that had been initiated by the British Government at an orphanage in Harpenden, England. Every 3 months, Tanner took photographs of the children in the nude as they progressed through puberty. The photographs of each youth were then combined, allowing for easy recognition of the stereotypical sequence of secondary sexual development that occurred in both girls and boys. He categorized this sequence into five phases, which became known as the Tanner stages [48, 49] and which quickly became the gold standard for the evaluation of pubertal development worldwide. Tanner also drew attention to the tremendous individual variation in rates of physical development, establishing the concept of “tempo” of puberty and to the distinct differences in the timing of pubertal onset between boys and girls. Indeed, we owe much of the bread and butter of pediatric endocrinology to the meticulous observations of Tanner and his PhD colleague Dr. William Andrew Marshall.

Through the years, different terminology depicting premature central pubertal onset has been in vogue. Early reports refer to “constitutional sexual precocity” or “constitutional isosexual precocious puberty.” The qualifiers “true” or “complete” were used to differentiate this entity from common variations of normal such as premature thelarche and premature adrenarche. “Gonadotropin-dependent precocious puberty” was and still is utilized to distinguish the condition from forms of peripheral (gonadotropin independent) precocious puberty. While all of these terms refer to the same physiologic phenomenon, and in all due respect to those who came before us, it is the authors’ humble opinion that the most descriptive and the least obtuse and convoluted term is “central precocious puberty” (CPP), and this is how we will refer to the disorder from this point forward.

Until the 1980s, there were three main pharmacologic approaches for the treatment of CPP. The first of these was medroxyprogesterone acetate (MPA), which was first reported as a potential therapy for the condition in the form of a Letter to the Editor by an ob/gyn physician in 1962 [50]. MPA quickly became adopted as standard of care by pediatric endocrinologists around the globe. It was typically administered either orally at a dose of 10 mg per day or in the form of an intramuscular injection of 100–150 mg every 2 weeks. MPA decreased gonadotropin levels to a variable degree in both boys and girls [51-53]. Girls usually experienced cessation of menses along with regression of both breast development and estrogenization of the vaginal mucosa. Unfortunately, however, there was minimal impact on the rate of skeletal maturation or on height velocity. This led some investigators to try higher doses of MPA in hopes of improving efficacy. In one report, four children with CPP received 200–300 mg of MPA intramuscularly every 7–10 days. While signs of secondary sexual development regressed, the therapy did little to slow linear growth acceleration or bone age advancement. Even more distressing was the development of adrenal insufficiency and cushingoid features in every one of the patients treated [54]. The ultimate conclusion was that there was a great need for a more effective treatment. The second medication that was tried in children with CPP was danazol, a synthetic 2,3-isoxazol derivative of 17α-ethinyl testosterone. The initial report, published in 1975, involved 3 girls and 2 boys. While sex steroid concentrations decreased during treatment with danazol, LH was inconsistently suppressed and no effect on FSH was seen [55]. Similar to MPA, danazol did little to ameliorate the linear growth acceleration and rapid skeletal maturation that occurs in CPP. Moreover, it resulted in virilization in girls, particularly at doses above 150 mg/m²/day. The third pharmacologic agent that was used, with great initial enthusiasm, was cyproterone acetate (CA), a synthetic steroid with antigonadotropic and antiandrogenic properties. Due to a beneficial effect on clinical and biochemical indices of pubertal progression as well as on growth rates and skeletal maturation, CA was touted as the treatment of choice for CPP [56]. Although early reports of the use of CA in children with CPP claimed that side effects were nonexistent [57], it subsequently became evident that a clinically significant degree of primary adrenal insufficiency resulted from this therapy [58, 59]. Thus, each of the agents available for the treatment of CPP in the 1960s and ‘70s was marred by serious flaws in terms of safety, efficacy, or both.

In 1981, the use of a long-acting GnRHa in a 2-year-old girl with idiopathic CPP was reported [60]. After 8 weeks of treatment, day and nighttime pulsatile secretion of gonadotropins was obliterated and estradiol fell to undetectable levels, confirming the phenomenon of “desensitization” that had been observed in animal studies. Treatment was shown to be fully reversible, and no adverse effects were noted. This was followed by a landmark paper later that same year that appeared in the New England Journal of Medicine reporting the use of leuprolide, a long-acting GnRHa, in 5 girls with CPP [61]. The medication, which was administered in the form of daily subcutaneous injections, significantly lowered basal and stimulated gonadotropins as well as serum estradiol. Eight weeks after stopping the GnRHa, all hormonal values reverted to pre-treatment levels, again demonstrating the reversibility of this novel therapeutic strategy. While short-term administration of GnRHas in children with CPP appeared to be highly efficacious and safe, a subsequent report demonstrated sustained biochemical suppression of the H-P-G axis in 9 girls treated for 18 months [62]. Moreover, treatment resulted in a decrease in height velocity, attenuation in the rate of skeletal maturation, and an increase in predicted adult height. These findings were corroborated in a subsequent study conducted in 19 children that included boys as well as girls and patients with organic as well as idiopathic etiologies [63]. In addition to favorable effects on growth, bone age X-rays and secondary sexual development, pelvic ultrasounds demonstrated regression of uterine and ovarian volumes from pubertal to prepubertal dimensions in most treated girls. These groundbreaking reports ushered in the era of GnRHas for the treatment of CPP worldwide. While early GnRHas required daily subcutaneous or intranasal administration [64], longer-acting depot intramuscular preparations were quickly developed [65]. In the years since, a plethora of even longer-acting extended-release GnRHa formulations have become available, ranging in potency from 20 to 210 times that of native GnRH [66]. A common feature of the pediatric preparations of GnRHas is their cost, rendering these medications hugely profitable for the pharmaceutical companies that produce them. A timeline illustrating the chronology of the various treatment options for CPP is seen in Figure 3.

In recent years, there has been a proliferation of commercially available GnRHas that have received regulatory approval for use in children with CPP. These analogs all include substitution of the naturally occurring L-glycine at position 6 in the decapeptide with a D-isomer amino acid. In some analogs, the 10th amino acid is deleted, with modification of the naturally occurring L-proline at position 9. These substitutions interfere with the sites of peptidase action and prolong the molecules’ half-lives (Fig. 4). In the USA, four analogs are currently available. Many of these GnRHas are also marketed for CPP under different trade names in Europe and other parts of the world. Leuprolide is marketed as monthly intramuscular injections at doses of 7.5 mg, 11.25 mg, and 15 mg. Leuprolide is also sold as every-three-month injections of 11.25 mg or 30 mg and as a subcutaneous injection given every 6 months at 45 mg. Leuprolide for CPP is marketed in the USA as Lupron® and Fensolvi®. Triptorelin is approved for the use in CPP at a dose of 22.5 mg intramuscularly every 6 months, sold as Triptodur®. Nafarelin is sold under the brand name Synarel® and is administered intranasally twice daily at 800 µg. Supprelin LA® is a subdermal implant containing histrelin and is FDA approved for yearly placement, although a clinical trial has shown it to last for up to 2 years [67]. Goserelin is available outside the US for the treatment of CPP under the trade name Zoladex® and is administered as a 3.6 mg monthly injection. These GnRHas are also sold under different names and at different doses for other conditions, such as breast cancer, endometriosis, and prostate cancer. Clinical trials aimed at adding additional options to the GnRHa therapeutic armamentarium continue to this day.

One of the most exciting breakthroughs in the realm of CPP has been the discovery of distinct monogenic causes of the condition, which currently number four. In 2008, an activating mutation in the kisspeptin receptor gene GPR54 was reported in an 8-year-old adopted Brazilian girl with CPP [68]. Functional studies revealed a delayed rate of degradation of the mutated protein, which was presumed to form the basis for prolonged intracellular signaling leading to early activation of the H-P-G axis. Two years later, a gain of function mutation in the KISS1 gene was reported in a boy with onset of CPP at 1 year of age [69]. Similar to what was observed with the mutant kisspeptin receptor, the abnormal variant was shown to be more resistant to degradation in human serum than the wild type. Additional commonalities between these two mutations include that they are heterozygous and have thus far been reported in only 1 patient each. The next genetic etiology of CPP to emerge was in the form of four novel heterozygous mutations in MKRN3, a maternally imprinted, paternally expressed gene encoding makorin RING-finger protein-3 [70]. The original report, published in 2013, involved 15 families with multiple affected individuals. MKRN3 mutations were identified in several members from five kindreds, all of whom inherited the defect from their fathers. MKRN3 mutations have subsequently been shown to be the most commonly identified genetic cause of CPP, accounting for up to 19% of familial and 2% of sporadic cases [71]. Based on studies performed in both mice and humans [72, 73], it is well established that MKRN3 levels decline as pubertal onset approaches, implying that this protein exerts an important inhibitory influence on the reproductive axis. The fourth molecular genetic abnormality identified in patients with CPP is deletions in DLK1, which encodes delta-like 1 homolog, also known as preadipocyte factor 1 [74]. Similar to MKRN3, DLK1 is also an imprinted gene that is expressed only from the paternal allele. Four female members of an Afro-descendent Brazilian family were found to have deletions of DLK1 with concurrent obliteration of circulating DLK1 levels. An additional role for DLK1 has been suggested by the observation that women with a history of CPP and a DLK1 deletion exhibit a distinct metabolic phenotype marked by higher rates of obesity, dyslipidemia, insulin resistance, and PCOS than controls [75]. Thus, DLK1, which has been referred to as a molecular gatekeeper of adipogenesis, may be an important link between reproduction and metabolism. Table 1 provides a summary of the genetic causes of CPP that have been elucidated thus far. Several additional putative genetic etiologies of early puberty exist but have yet to be identified in any children [76].

In conclusion, the past century has witnessed extraordinary progress in the elucidation of the physiologic underpinnings of the reproductive system. Exciting molecular genetic discoveries have partially illuminated “the black box of puberty,” yielding novel insights into both normal and precocious development. Ongoing advances in treatment have greatly expanded the therapeutic armamentarium for children with CPP. It is intriguing indeed to ponder what new developments may emerge in the next 50 years!

The authors Drs. Fuqua and Eugster declare no conflict of interest.

No funds were received for this review.

Both authors Dr. John S. Fuqua and Dr. Erica A. Eugster contributed equally to the conception and production of the manuscript.

No new data were generated during the production of this manuscript.